From: David Smith <dsmith@redhat.com>
To: Roland McGrath <roland@redhat.com>
Cc: Maynard Johnson <maynardj@us.ibm.com>,
systemtap@sourceware.org,
"Frank Ch. Eigler" <fche@redhat.com>
Subject: Re: Backward compatibility for insn probe point
Date: Thu, 14 May 2009 15:11:00 -0000 [thread overview]
Message-ID: <4A0C347D.9060204@redhat.com> (raw)
In-Reply-To: <20090513182359.0CF33FC35D@magilla.sf.frob.com>
Roland McGrath wrote:
>> I poked around the kernel source some more, but couldn't see what was
>> going wrong.
>
> I figured you'd use some stap probes to follow the code paths!
Oh I did, but so much of it is inlined that it was incredibly difficult
to follow.
>> I've changed the itrace code to stop the task after each step trap (so
>> that it acts more like ptrace). I've tested this on several kernels
>> (2.6.18-141.el5/ppc, 2.6.18-128.1.10.el5/x86_64/i686, and
>> 2.6.25-14.fc9.ppc64) and it seems to work correctly.
>>
>> Does this seem like a reasonable work-around? Could there be problems
>> with this approach?
>
> I presume it kills performance. But what works works, that's a what a
> work-around is. I'd hope that you don't make it use this "not really
> right" mode for kernels with the modern utrace interface that doesn't have
> this bug.
Yes, this is only for old utrace. As far as performance goes, I've
benchmarked single-stepping '/bin/ls' on x86_64 with both approaches.
Here's what I saw ('time' output, averages of 5 runs):
- no stopping on each step trap:
real 0m3.735s
user 0m0.328s
sys 0m3.359s
- stopping on each step trap:
real 0m4.101s
user 0m0.336s
sys 0m3.692s
I could also limit this work-around to ppc-only, to not penalize other
architectures.
One last thing. I thought I'd try block stepping, so I got access to an
ia64 machine. Unfortunately, using systemtap insn probes (either single
or block step) lock up the system with a spinlock lockup. Sigh.
--
David Smith
dsmith@redhat.com
Red Hat
http://www.redhat.com
256.217.0141 (direct)
256.837.0057 (fax)
next prev parent reply other threads:[~2009-05-14 15:11 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2009-04-01 21:59 Maynard Johnson
2009-04-27 21:01 ` David Smith
2009-04-29 13:56 ` Maynard Johnson
2009-04-29 21:08 ` Maynard Johnson
2009-04-30 18:00 ` David Smith
2009-04-30 20:48 ` Roland McGrath
2009-05-12 16:06 ` David Smith
2009-05-12 18:20 ` Roland McGrath
2009-05-13 15:04 ` David Smith
2009-05-13 18:24 ` Roland McGrath
2009-05-14 15:11 ` David Smith [this message]
2009-05-14 18:41 ` Roland McGrath
2009-05-14 19:01 ` Maynard Johnson
2009-05-14 19:44 ` David Smith
2009-05-14 21:36 ` David Smith
2009-05-15 13:52 ` ia64 hang when using itrace (was Re: Backward compatibility for insn probe point) David Smith
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